Question

How does a star life cycle begin?

Answer 1

Explanation:

All stars go trough a lifecycle and it's life cycle is determined by its MASS – The larger the star, the faster it burns out! The star’s MASS is determined by the MATTER available in the nebula of formation.
The life cycle depends on size of
a) Sun-like stars, up to 1.5X mass of the Sun
b) Massive stars, (1.5 - 3)X mass of of the Sun
c) Super Massive, larger that 3X mass of Sun

Let's use the picture as a reference:
`color(brown)("Type a) star like Sun")`
Stage1 - Nebula - `color(violet)(Stellar Nursery)`
1) All stars start as a Nebula . A nebula is effectively a stellar nursery
Stage2 - Protostar formation - `color(blue)(Fetus)`
2) A region of condensing matter will begin to heat up and start to glow forming Protostars. In human terms this would be he `color(blue)(Fetus)`. If a protostar contains enough matter the central temperature reaches 15 million degrees centigrade.
Stage3 - Main Sequence `color(purple)("Infancy-Adulthood")`
3) At this temperature, nuclear reactions in which hydrogen fuses to form helium can start.
4) The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star.
The nearest main sequence star to Earth, our own Sun.
5) A star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium.
6) The helium core now starts to contract further and reactions begin to occur in a shell around the core.
Stage4 - Red Giant Formation `color(red)("Middle Age")`
7) The core is hot enough for the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a `color(red)(Red)` Giant.
Stage5 - Planetary Nebula
8) The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula.
Stage6 - White Dwarf Formation `color(grey)("Old Age/Death")`
The remaining core (thats 80% of the original star) is now in its final stages. The core becomes a `color(grey)(White)` Dwarf the star eventually cools and dims. When it stops shining, the now dead star is called a `color(black)(Black)`Dwarf.

`color(brown)("Type b) and c) massive star and super massive")`
The Stages for b) and c) are similar to the above until "Stage4"

Stage5 -
7) The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas.
8) The massive star is much bigger in its expanding stage.
9) In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.

Stage 6 - Supernova
10)The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than the entire galaxy for a short time

Stage 7 - Death
11) Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.